The present invention relates to the manufacture of plates and crockery of fine ceramic, such as,porcelain, and more particulary to the to so-called isostatic methods and apparatus therefor.
The isostatic method for dry-compacting powders uses so-called isostatic moulds in which a base has the shape of one side of the plate, against which there rests an elastomeric membrane of, substantially constant thickness, sealedly fixed to the perimeter thereof.
The base of the mould is positioned in front of a movable punch of the same shape as the other side of the plate.
The various stages in forming the crude plate, i.e. making it ready for firing in a suitable kiln, comprise arranging a layer of soft ceramic powder on a base, moving a punch towards the base until it closes define a, compartment formed between the base and the punch, without exerting substantial pressure on the powder, and introducing a pressurized liquid against the rear of an elastomeric membrane which covers the base, with consequent raising of the membrane from the base on which it rests, and compression of the material to achieve the compaction required to form the plate.
Subsequent withdrawal of the punch makes the plate accessible for its removal, after which the cycle is repeated.
In the known art the aforedescribed basic process can undergo various modifications in terms of the method of feeding the powder, the method of removing the plate, the positioning of the mould with its axis horizontal or vertical, the method of fixing the membrane, etc.
The aforedescribed cycle stages are however common to all known isostatic methods, and will hence be taken as the starting point in describing the present invention.
With the improvements in material technology and the ongoing increase in the dimensions of objects to be formed, increasingly greater forming pressures are required, with consequent increase in press dimensions and capacity.
The object of the present invention is to achieve material compaction suitable for modern technological requirements while maintaining the pressure of the membrane operating liquid relatively low, and hence maintaining low stress forces on the pressed structure and a low required capacity for the pressurized hydraulic liquid generator.
This object is attained according to the present invention by subjecting the membrane pressing against the powder under compaction to a certain number of pressure pulses while being subjected to the forming pressure. It has been found that the pressure pulses to which the membrane is subjected increases the powder densifying effect compared with the densification obtained by applying the same pressure as done in traditional methods.
The number of pressure pulses to be imposed on the membrane is conveniently less than 50 per cycle, it having been found that after a certain number of pulses a saturation effect occurs such that compaction is not further, substantially increased.
The number of pressure pulses can also be uncontrolled, if the pulses are generated by a vibrator or by an ultrasound generator in contact with the hydraulic liquid.
It has also been found that the effect of pressure pulses increases as a function of the base pressure applied, in the sense that whereas for low pressure, of the order of 100 bar, compaction increases by a certain amount, for higher base pressures the increase in compaction is comparatively much greater.